that's because there was an axle (model APL325) that was used 1981 thru 4/84 then the one you're looking at began.
I worked as a parts mgr 84-98 and I promise you that you can't make rhyme or reason out of a Ford parts book, especially
after the first model the book was written for has running changes like the front end change. They weren't willing to completely re-write the books so they were forced to add pages that really don't make sense. you need to look at your axle casting real good, you will find a model number on it that begins APLXXX so you know which pages to
look at. The link I posted above is for the shield for the years 1981/4-1984